At the moment, accumulating 51% of the Bitcoin network's hash rate is prohibitely expensive. An attacker would be better off just using that massive hash power to legitimately mine bitcoins. This is the game theory behind bitcoin mining.

Having said that, smaller PoW coins have been recently hit by 51% attacks, as renting the needed hash power for them is cheaper than the profits from selling the double spent coins.

Not too expensive if you're Beijing. 75% of hashpower is centralized within the PRC. All Beijing has to do is inform the miners they now do their bidding. This kind of thing happens all the time in the PRC. The 'community' lost this fight ages ago, at this point Bitcoin effectively operates at the pleasure of Beijing, and one day that may very well change.